1. Field of the Invention
The present invention relates generally to a method of manufacturing an acceleration sensor, and more particularly, to an acceleration sensor manufacturing method suited to detect a large variation in velocity which is caused in the event of a collision of a vehicle.
2. Description of the Prior Art
This type of acceleration sensor was disclosed in U.S. Pat. No. 4,827,091. This sensor includes: a cylindrical body formed of a conductive material; a magnetized inertial body so positioned in an interior of the cylindrical body as to be movable in the longitudinal direction of the cylindrical body; a conductive body provided on an end surface of at least one end of the magnetized inertial body in the longitudinal direction of the cylindrical body; a pair of electrodes disposed at one end in the longitudinal direction of the cylindrical body and made conductive through the conductive body when contacting the conductive body of the magnetized inertial body; and an attracting body composed of a magnetic material, disposed at the other end in the longitudinal direction of the cylindrical body and magnetically mutually attracting the magnetized inertial body.
In this acceleration sensor, the magnetized inertial body and the attracting body attract each other. When absolutely or almost no acceleration is applied to the acceleration sensor, the magnetized inertial body is stably situated at the other end of the interior of the cylindrical body.
If a relatively large acceleration is applied to this acceleration sensor, the magnetized inertial body moves while resisting the attracting force associated with the attracting body. When the magnetized inertial body is going to move, an induced current flows in this cylindrical body. A magnetic force for biasing in a direction opposite to the moving direction is imparted to the magnetized inertial body, and it follows that the magnetized inertial body is brought into a braked state. A moving velocity thereof is decreased.
If the acceleration is smaller than a predetermined value (threshold value), the magnetized inertial body does not reach the top end of the cylindrical body. Instead, the magnetized inertial body stops midway and is subsequently returned to the other end by the attracting force associated with the attracting body.
Whereas if the acceleration is greater than the predetermined value (threshold value) (i.e., in the event of a collision of a vehicle mounted with this acceleration sensor), the magnetized inertial body reaches one end of the cylindrical body. A conductive layer on the top end surface of the magnetized inertial body contacts a pair of electrodes. The electrodes are thereby made conductive to each other. If a voltage is applied beforehand between the electrodes, the current flows in between the electrodes just when the electrodes short-circuit each other. The collision of the vehicle is detected from this current.
In the case of manufacturing the acceleration sensor in the prior art, the previously magnetized inertial body has hitherto been inserted into the cylindrical body. This method, however, presents a variety of drawbacks to the assembling operation in which the inertial body to be assembled attracted to an iron tool or operating board in the manufacturing work place because of the inertial body being magnetized.